The drain size is based on how much water COULD be flowing into the shower. In your case, since theoretically, there could be more water than a 2" drain could handle, I would opt for a 3" drain, rather than two 2" ones. The actual water flow will be determined by the capacity of the shower valve AND which flow controls are open at any one time.

I'm not familar with code on such things, but in designing process equipment for self venting flow (so that the drains don't back up) a 2" pipe is only good for about 8 gpm. Your design is for 12.9 gpm when all are running. So I agree with the inspector that a single 2" drain is insufficient.

If the two 2" drains run together as a single 2" line, then it doesn't improve the flow at all since functionally, all you have is a 2" drain. It only works if you run each 2" drain to a larger pipe separately. Otherwise, make it a single 3" drain.

If the valves are diverters and you can't run all of the heads at the same time, the story would be different. BUT, if they all CAN be on at the same time, you may have problems. Note, you'd need 3/4" valve minimum to achieve those flow rates, and you may still notice performance issues since the valve would be close to being maxed out.

Two 2" drains into a 2" drain pipe does not make much sense if added flow is the reason for them, ESPECIALLY if there are ANY other drains connected to the 2". The flow capacity for a pipe is based on both the horizontal and vertical components, not just the pipe from the drain to the trap.

With fixtures less than 2.5GPM each, and body sprays less than 1.8GPM each, you might save yourself a lot of work by using your existing pipes instead of redoing them. Make the total match a two inch drain. There is a maximum flow in any size of pipe.

What distance is between the shower drain p trap and the next bigger pipe?
What distance is there until venting?

Did you say in your first post that there is an overhead shower and a fixed shower, and a handshower? What is the difference between the first two?

Edited to add this: Also, behind the DFU numbers are arithmetic calculations that you can do, too. DFU tables are shortcuts that work well except in this kind of situation, which is more complex than was foreseen by the framers of the DFU tables.